Dear Carole, Why does my girlfriend enjoy being tied up? Sex always involves rope, handcuffs or restraints of some sort, and often uniforms. She is anything but submissive outside the bedroom, but seems unable to achieve orgasm without being restrained in some way.

When we have "vanilla" sex she enjoys it, but when tied she always has multiple orgasms. While I'm not complaining at her tastes, I'm not sure I can see the evolutionary benefit … apart from attracting men who are dominant in bed, which, at the risk of doing myself down, seems a dubious sort of goal at best.

(If it's all the same with you, in the event that you publish this, I'd rather remain anonymous: she reads the Guardian too.)

Carole replies:

Public Domain

Are you dominant in bed? Once an adult is handcuffed and restrained the other party becomes, by default, the dominant one. You say that outside of the bedroom your girlfriend is "anything but submissive". Is it possible, therefore, that in the bedroom it is actually you who is dominated? After all, it is her desire and not yours, for a ritualised, fetishised sex that evidently dominates your sex life.

Orgasm

Non-human primate females commonly experience multiple orgasms during sex1. Within a short space of time, oestrus females can have sex with many partners and experience cumulative orgasms. Any number of males could be the father of the resulting infant and with a mix of sperm from various males in her reproductive tract the strongest swimmer makes it to the ovum first. Many primate species, including humans, are sexually dimorphic; frequently the males are larger and more powerful. During a process of multiple matings the oestrus female may be raped by competing males she does not favour; yet she may experience an orgasm and an infant may also result.

Western science finally accepted that women experienced orgasms in 19662. It was then theorised the female orgasm was uniquely human and evolved to encourage women to bond to their men. Until the early 1970s it was widely assumed, by male primatologists, that the non-human female primate did not experience orgasm. But women field primatologists studying wild apes and monkeys interpreted the sexual physiology of the female primate differently and accepted that orgasm occurs naturally and did not need to be proved. Instead, they were interested in why the female orgasm had evolved.

To settle the debate laboratory-based male scientists strapped captive female monkeys into ankle grips and dog harnesses3. They then stimulated the animal's clitoris and inserted an instrument, in place of a male macaque penis, into the animal's vagina. They observed the female monkeys experienced orgasm similarly to women. Three stages were noted, firstly an excitement phase in response to clitoral and vaginal stimulation, secondly a plateau phase where the monkey grunts, calls, makes an open mouthed face and reaches out, and finally a resolution phase, post orgasm, where the female tries to escape.

It has been suggested that sperm retention takes place via the biological function of the female orgasm aiding that particular male's chances of fertilising the ovum4.

Testosterone

It has been theorised that rape is an evolved reproductive strategy in male primates, including humans5. From DNA data it has been estimated that 13th century Mongol emperor and war lord Gengis Khan has approximately 16 million male descents alive today6. Gengis and his sons and grandsons acquired thousands of women from conquered territories and enslaved them as concubines. Clearly those women did not choose Gengis, he chose them for their looks and virginity. But today, more than a thousand years later, because they were sexually dominated by a powerful man, the genes of the women who produced sons like Gengis proliferate in the region of the former Mongol empire.

During the first half of the menstrual cycle women's testosterone levels increase. By the time they ovulate their testosterone peaks and during the latter half of the cycle testosterone falls.

Testosterone levels help to promote sex drive in both males and females, women's sexiness coinciding with their most fertile time. Testosterone levels vary in women as they do in men.

Research has shown that women higher in testosterone are more attracted to more macho behaviour in males7 8.

You don't say that your girlfriend wants you to restrain her and dominate her during her most fertile time of the month, but this may be the case. It may also be the case that your girlfriend has above average levels of testosterone, which may help facilitate a successful dominance in other spheres of her life.

But her dominating personality may prevent her from seeing herself as innately sexy or as the cultural stereotype of "feminine". It is possible for her to achieve a self-image as an alluring sexy female, she may (perhaps wrongly) believe she needs to play a submissive role. Studies on the sex lives of female primates have revealed that female homosexual behaviour is frequently favoured and that female primates can dominate sexual activity by initiating hetero and homosexual coitus, partaking in varied couplings and positioning and invariably experiencing orgasms9.

Prelapsarian sex

The paraphernalia of handcuffs, rope and blindfolds used by modern humans during sex inflicts passivity and a removal of freedom of choice upon the restrained one. For self-reflective sophisticated communicators, which most humans are, sex of this sort becomes reductive and objectified.

It is possible that at these times your girlfriend experiences sex with you via the third person objective. Your focus on her external appearance is realised by her, via your gaze upon her, where you, your body, your mind and your objectification of her all become essential elements of the very common sex game being played out privately in your safe bedroom.

For some women this physiological process may evoke primal, primitive reflexes and consenting human adults may be able to psychologically simulate something similar to the emotional content of prehistoric animal sex. With one party blindfolded or gagged communication channels, in terms of mind-reading or the spoken word, are reduced.

We separated from ancestral monkeys (new and old world) some 30m to 40m years ago. The female orgasm predates the evolution of spoken language and our prehistoric female monkey ancestors were experiencing cumulative orgasms long before human self-reflection and self-awareness evolved.

Sexual submission is common occurrence in the natural world of wild primate females. Your girlfriend is not submissive, nor is she without female choice as you are her partner of choice, but for a short fantasy period she chooses to reverse her role in life into one of sexual subjugation. Because this sex game is repetitive she has learned through practice (possibly with previous boyfriends) that it will give her sexual satisfaction.

If I were you, instead of asking, "why does my girlfriend enjoy being tied up?", I might question, when those handcuffs come out, whether it's really me she wants. Or is she just feeling horny and in need of a male to animate her reductive, prelapsarian fantasy?